1. Technical Field
The present invention relates to an ultrasonic diagnostic apparatus, and, in particular, to a technique for forming an ultrasound beam using an array transducer.
2. Background Art
Two-dimensional array transducers constructed by two-dimensionally arranging a plurality of transducer elements are known. The two-dimensional array transducer is formed, for example, with a few thousand transducer elements which are electrically controlled. With the two-dimensional array transducer, ultrasound beams are two-dimensionally scanned, and echo data are three-dimensionally collected.
When a plurality of transducer elements which are a part of the two-dimensional array transducer are controlled, if a signal line is provided independently for each transducer element, a number of signal lines equaling the total number of transducer elements; for example, a few thousand signal lines, would be required for the overall two-dimensional array transducer. When a few thousand signal lines are employed, a probe cable connecting a probe in which the transducer elements are disposed and a device body would become thick, and the number of pins of the connector in a connection portion between the probe cable and the device body would also become large. In addition, the increase in circuit size for transmission/reception systems cannot be ignored. In consideration of these circumstances, techniques have been proposed for reducing the number of signal lines (number of channels) connecting the plurality of transducer elements and the device body.
For example, Patent Literature 1 (JP 2001-286467 A) discloses a technique in which a focus point of an ultrasound beam is projected onto an array transducer surface, a plurality of concentric rings are set centered at the projected point, and a common amount of delay is correlated to a plurality of transducer elements belonging to each concentric ring. With such a configuration, the number of channels can be significantly reduced to the number of concentric rings. However, a switching circuit for electrically collecting the plurality of transducer elements belonging to each concentric ring becomes very complex, and the amount of data for controlling the switching circuit also becomes very large.
Patent Literature 2 (JP 2005-342194 A) discloses a technique in which the two-dimensional array transducer is divided into a plurality of sub-arrays, and a plurality of transducers corresponding to the same amount of delay in each sub-array are connected (grouping). With the employment of the sub-array as in Patent Literature 2, although the number of channels is increased as compared with the case where the sub-array is not employed (for example, configuration in Patent Literature 1), the size of the switching circuit can be reduced as compared with such a case. However, the amount of data for controlling the switching circuit is still enormous.
In order to electrically collect a plurality of transducer elements by a switching circuit, for example, information for controlling the switching circuit is supplied from the device body to the probe. In order to control the switching circuit according to a steering angle of the beam and depth of focus, the amount of information to be supplied from the device body to the probe becomes very large. For example, when information related to a few thousand transducer elements is transferred, a transfer time of a few tens of microseconds would be required, resulting in other problems such as reduction in the frame rate.
Patent Literature 3 (JP 2000-33087 A) discloses a technique to apply a control such that the depth of focus is fixed at infinity and only the beam steering is considered, in order to simplify the beam control and reduce the amount of information. However, when the depth of the focus is simply set to infinity, converging of the beam etc. is degraded and the precision of the beamforming is reduced, resulting in reduction in resolution or the like of the image.
Under such circumstances, the present inventor have researched and developed a technique which inhibits degradation of the ultrasound beam while inhibiting an increase in the amount of information for controlling the ultrasound beam. In particular, the present inventors have focused attention on a grouping process for collecting a plurality of transducer elements of the array transducer into a plurality of element groups.